Many species of snakes have skulls with several more joints than their lizard ancestors and relatives, enabling them to swallow prey much larger than their heads cranial kinesis. To accommodate their narrow bodies, snakes' paired organs such as kidneys appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.
Lizards have independently evolved elongate bodies without limbs or with greatly reduced limbs at least twenty-five times via convergent evolution , leading to many lineages of legless lizards. Living snakes are found on every continent except Antarctica, and on most smaller land masses; exceptions include some large islands, such as Ireland, Iceland, Greenland, and the islands of New Zealand, as well as many small islands of the Atlantic and central Pacific oceans.
Around thirty families are currently recognized, comprising about genera and about 3, species. The oldest preserved descriptions of snakes can be found in the Brooklyn Papyrus. Most species of snake are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom that is potent enough to cause painful injury or death to humans.
Nonvenomous snakes either swallow prey alive or kill by constriction. All modern snakes are grouped within the suborder Serpentes in Linnean taxonomy , part of the order Squamata , though their precise placement within squamates remains controversial.
The two infraorders of Serpentes are Alethinophidia and Scolecophidia. Alethinophidia is sometimes split into Henophidia and Caenophidia , with the latter consisting of "colubroid" snakes colubrids , vipers , elapids , hydrophiids , and atractaspids and acrochordids, while the other alethinophidian families comprise Henophidia.
